1. Field of the Invention
The present invention relates generally to a TD-CDMA (Time Division-Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for enabling a user equipment (UE) to detect downlink channelization codes assigned to other UEs.
2. Description of the Related Art
Recent developments in the communications industry have broadened the range of services available in mobile communication systems from voice service to multimedia communication service in which a large volume of data such as packet data or multimedia data is transmitted. More specifically, downlink traffic is increasing relative to uplink traffic. Also, TDD (Time Division Duplex) is a more efficient scheme than FDD (Frequency Division Duplex) in supporting a multimedia communication system. The TDD identifies the uplink and the downlink by time slots, whereas the FDD identifies them by frequencies.
A common FDD system is IS-95A/B, which is based on CDMA, and a common TDD system is GSM (Global System for Mobile communication), which is based on TDMA (Time Division Multiple Access). Efforts have been made worldwide to develop 3rd generation (3G) communication systems from the 2nd generation (2G) communication systems. As a result, WCDMA (Wideband CDMA) and cdma2000 have been adopted as international FDD standards, while TD-CDMA has been adopted as an international TDD standard. TD-CDMA is an expansion of the existing TDD scheme, in which a plurality of code channels is assigned to a time slot.
There are two TD-CDMA systems: an HCR-TDD (High Chip Rate-TDD) that supports a 3.84-Mcps chip rate and an LCR-TDD (Low Chip Rate-TDD) that supports a 1.28-Mcps chip rate. A TD-CDMA system uses up to 16 channelization codes in each time slot. If a spreading gain of one time slot is 16, up to 16 code channels are used in the time slot.
For coherent reception, the TD-CDMA system uses a midamble sequence (or a training sequence), which is similar in function to the pilot channel of the CDMA system. A TD-CDMA receiver estimates a channel characteristic (impulse response) using the midamble sequence inserted in data. Based on the channel characteristic, the TD-CDMA receiver simultaneously eliminates through a joint detector (JD) multipath-incurred intersymbol interference (ISI) and multiple access interference (MAI) caused by communication accesses from multiple users.
A basic joint detection algorithm for the uplink will first be described herein below. Each of K UEs uses one channelization code and the spreading gain of the channelization code of a kth UE is Q. The channelization code of the kth UE having the spreading gain Q is denoted by c(k) and the impulse response of an uplink radio channel from the kth UE to a Node B is denoted by h(k). If the radio channel has a time delay of (W-1) chips, its impulse response is expressed as h(k)=[h1(k),h2(k) . . . hw(k)]T, where T denotes transpose.
For N data symbols d(k)=[d1(k),d2(k) . . . dN(k)]T transmitted from the UE by its channelization code, the Node B receives a signal e=[e1,e2, . . . , eN·Q+W-1]T that can be expressed as e=Ad+n. Then, the N data symbols can be expressed as a column vector b=[d(1)T,d(k)T . . . d(k)T]T. Here, n is an AWGN (Additive White Gaussian Noise) vector of length NQ+W-1 and the elements of A are a combination of the channelization code c(k) and the impulse response h(k).
ZF-BLE (Zero Forcing-Block Linear Equalizer) is a common joint detection algorithm. The ZF-BLE algorithm estimates data by multiplying both sides of e=Ad+n by (AHA)−1AH. That is, {circumflex over (d)}=(AHA)−1AHe=(AHA)−1AH(Ad+n)=d+(AHA)−1AHn is taken as a symbol estimate. Superscript H denotes complex transpose. By excluding an AWGN-caused estimation error using the ZF-BLE algorithm, both MAI and ISI are eliminated at the same time.
The above joint detection offers excellent performance compared to an existing rake receiver, albeit its complexity. This advantage renders the joint detection almost essential to TD-CDMA systems using small spreading factors. In regard to the joint detection, therefore, extended algorithms to multiple antenna systems, MIMO (Multiple Input Multiple Output) systems, and smart antenna systems as well as many algorithms for reducing complexity with respect to performance have been proposed successively.
Implementation of the joint detection requires that a UE know channelization codes used for other UEs to eliminate MAI caused by the channelization codes. On the uplink, the Node B controls radio resources within its cell area and thus accurately knows channelization codes assigned to UEs within the cell area, which eliminates the need for exchanging additional channelization code information to eliminate ISI and MAI by joint detection. That is, because the Node B already has knowledge of the channelization codes (hereinafter, referred to as active channelization codes) assigned to the UEs within its cell area, it can accurately recover uplink data from them.
On the downlink, however, a UE has no knowledge of channelization codes assigned to other UEs. Therefore, the Node B notifies the UEs within its cell area of the active channelization codes, or each UE acquires information about the channelization codes on its own in order to eliminate MAI by joint detection. Active channelization codes are defined as a set of channelization codes in use for UEs within a predetermined cell area.
Traditionally, it was discussed that if a change occurs in an assignment of radio resources, the Node B notifies all active UEs within its cell area of changed active channelization codes. The radio resource assignment is changed when a handover between the Node B and a neighbor Node B, an internal handover within the Node B, a new call setup, or discontinuation of an ongoing call occurs. However, transmission of information relating to the changed active channelization codes to all the active UEs by signaling each time a change occurs in the radio resource assignment increases signaling load. As a result, there is a pressing need for a method that enables a UE to acquire active channelization code information without additional signaling.